A DC power source (a battery) is mounted, for example, on a vehicle, and the DC voltage of the DC power source is used as various kinds of powers and for various controls. This DC voltage of the DC power source is converted into a pulse row-like pseudo AC voltage using an inverter circuit with a plurality of switching elements, and for example, the converted pseudo AC voltage is used for a motor for driving a compressor in an air conditioner for a vehicle (for example, a brushless motor of an electric compressor, etc.) or another motor (for example, a reluctance motor or an induction motor), and used also as an output voltage to a transformer and the like.
As a compressor used in an air conditioner for a vehicle (refrigerant compressor), although there is a compressor employing only rotational force of an engine as its drive source, as an electric compressor or a hybrid compressor, usually a compressor incorporated with a brushless motor is used. This brushless motor is driven by a pulse row-like pseudo AC voltage converted by an inverter circuit, and its rotational speed is controlled by a method for changing a supplied effective voltage obtained by changing the duty ratio of the above-described pseudo AC voltage, namely, PWM (pulse width modulation).
However, a DC power source such as a battery, etc., mounted on a vehicle is liable to generate a voltage fluctuation in relation to an electric load, and for example, in a vehicle such as an electric car or a hybrid car having a motor for running, because a great load current flows in the motor for running at the time of accelerating, the voltage of a DC power source is rapidly lowered, and on the other hand, at the time of decelerating, the voltage of the DC power source is rapidly elevated by a regenerated current from the motor for running. Further, even in a usual engine-driving vehicle, if a power of a DC power source is consumed at a great amount, the voltage of the DC power source may be rapidly lowered.
If such a fluctuation of a voltage of a DC power source occurs, in proportion thereto an output voltage fluctuates. For example, when the above-described brushless motor is driven, accompanying with the voltage fluctuation, the rotational speed of the motor may be rapidly accelerated or decelerated. Further, an excessive current may flow at the time of deceleration, etc., and at worst, a protection circuit may operate and the motor may stop.
In order to deal with such a problem, in Patent document 1, a technology is disclosed wherein a voltage of a DC power source is detected and the duty is changed in accordance with the detected voltage value. Further, in Patent document 2, a technology is disclosed wherein a voltage of an output from an inverter circuit to a motor is detected and the duty is changed in accordance with the detected voltage value.    Patent document 1: JP-B-3084941    Patent document 2: JP-A-2005-57974